A typical railcar includes a car body that rides on one or more railway trucks, also known as bogies. The trucks support the car body vertically and laterally while allowing sufficient rotational movement between the trucks and car body to allow negotiation of curved track.
The trucks are generally proximate to each end of the car body and support the car body for transport along the rail through a suspension system. Each truck generally includes a frame that connects two or more wheel-sets. The frame includes a pair of side frames that extend along the length of each side of the truck. A bolster connects the side frames to hold the side-frames generally parallel to one another.
In the case of freight car trucks in particular, it is common practice in North America, as well as in other jurisdictions, for the suspension system to consist of a set of steel coil springs supporting the load of the car body and an arrangement of springs and wedges to provide friction damping of both vertical and lateral motions of the car body.
A typical friction damper arrangement 1 is shown in FIG. 1. As can be seen, two vertical wear plates 2 are mounted in the bolster opening 3 of the side frame 4 and a pair of friction wedges 5 is pressed into contact with these plates 2. Sloping faces 6 on the wedges 5 contact similar sloping faces 7 on the bolster 8 and the action of springs 9 pressing the wedges 5 into contact with those sloping surfaces 7 causes the wedges 5 to press outwards against the vertical wear plates 2. Each side frame 4 is provided with this friction damper arrangement 1.
This arrangement is known as variable friction damping, since the forces on the wedges 5 from the springs 9 vary with the height of the bolster 8 within the bolster opening 3 and that, as a result of this, the friction forces between the wedges 5 and the wear plates 2 also vary.
There are two main variations of this type of arrangement. In the first of these variations, the springs are disposed between the wedges and surfaces of the bolster, and thus the spring forces do not vary as the height of the bolster varies. In the second of these variations, the bolster has a vertical face on each of its sides and the wedges act against sloping faces in the side frame. In this variation, since the springs still react against the side frame, there is no motion of the wedge as the bolster moves, and as a result, there is no variation of the spring and friction forces.
By arranging the wedges as mentioned above, a ‘squaring’ effect is obtained between the bolster and the two side frames of the railway truck. In many trucks, the only connection between one side frame and the other is that provided by the bolster, and the only connection between the bolster and the side frames is that provided by the wedges. The wedges are longitudinally spaced apart from each other as shown in FIG. 1 in order to prevent the side frame/bolster connection from rotating, thus allowing the side frames to move longitudinally relative to each other. The wedges also have a significant amount of pressure applied to them by the springs in order to prevent the undesirable rotation at that connection.
However, by using the wedges to accomplish tasks within the truck other than damping, their effectiveness as suspension dampers is compromised. The required spacing of the wedges, the forces applied to them, and their width, when sufficient to provide stability of the truck frame, create an undesirable level of resistance to truck vertical twist, thus compromising the equalisation of wheel loads, and provide excessive damping for the suspension, resulting in the suspension being ‘locked up’ for much of the time. These factors prevent truck designers from optimizing the damping qualities of the suspension and they degrade the safety and dynamic characteristics of the truck.
Also, with this type of arrangement, the ‘squaring’ of the truck varies with the level of wear at the wedges and the loads on the springs. This can have adverse effects on the ability of the truck to travel at high speeds and also on its ability to negotiate curved track. These factors combined with the above-noted adverse effect on the wheel vertical loads can create, in extreme cases, a situation where derailment of the truck can occur, adversely affecting the safety of the railroad operation.
Therefore, there is a need for a railway truck having a damping system which offers less resistance to truck vertical twist.